Re: General Design Questions...
Short answers: (1) The principal protection of TVSS belongs at the main panel, and supplemental protection can be added at the local panels, and (2) You can use both TVSS and ISO. Longer answers follow.
Transient Voltage Suppression Systems can protect against surges, as you have said. But they disregard the power distribution system at all other times. Absent a surge, they do nothing to monitor or alter the voltage or current waveform. (ASIDE - By contrast, several types of Uninterruptible Power Supply (UPS) Systems significantly alter the waveform. They take the incoming power, convert to DC, provide a backup battery system, and convert back to AC. The result is a far cleaner waveform, without regard to any anomalies in the incoming power system. END OF ASIDE) Lightning strikes and medium voltage switching (i.e., by the utility equipment) can cause surges. If you protect anything, you need to start by protecting the main switchboard. If the facility of interest has a large number of large motors, then their start and stop cycles can cause surges that might adversely impact other equipment in the building. Putting individual TVSS units at local panels can provide an additional level of protection against such internally-generated surges. It?s a matter of cost versus the reduction of risks, and is really the owner?s call.
An Isolated Ground System is a different animal. It consists of an Equipment Grounding Conductor (EGC) for the ?special equipment? that is separate from the EGC of all other equipment. The separate EGC is run all the way back to the point at which the service (or the separately derived system) has its connection to planet Earth. What this does is to prevent any variations in voltage within the EGC that serves ?common equipment? from influencing the ?zero reference point? upon which the proper operation of sensitive electronic equipment often depends. I am made to understand that the need for isolating the ground is becoming less and less important. However, I don?t know why this may be true, since the specialty of electronics is outside my interests. Someone else will have to take up the story from here.
